Generally, liquid spray recording apparatuses employ a liquid spray head comprising a liquid chamber, nozzles, liquid paths and an ink supply system. Such apparatuses are utilized by applying energy to the ink filled in the liquid chamber causing the ink stored therein to be ejected out through the liquid paths. As a result of this ejection, ink drops are sprayed from the nozzles, whereby character and graphic information or the like is recorded on a recording medium. A means that pressurizes the chamber, by way of example, a piezoelectric element or a heater for heating the ink in the liquid chamber, is used to apply the energy to the ink.
Conventional liquid spray heads similar to that described above and components related thereto are discussed in Japanese Patent Publication No. 62-22790, Japanese Laid-Open Patent Application 2-219654, U.S. Pat. No. 4,312,008, Japanese Journal of Applied Physics, Vol. 30, No. 12B, December 1991, pp. 3562-3566 by Torii, et al., Japanese Patent Publication No. 4-43435, and Japanese Laid-Open Patent Application 3-124450.
Japanese Patent Publication No. 62-22790 relates to a production method for a liquid spray head that forms an electrode on a substrate, which is fabricated as a thin layer in a location corresponding to the liquid chamber. A lead zirconate titanate (PZT) thin film is formed at a location corresponding to the liquid chamber by a sputtering, printing or other thin-film formation techniques.
In Japanese Laid-Open Patent Application 2-219654, a liquid spray head is provided with liquid chambers and liquid paths formed on a thin plate laminated on a semiconductor substrate provided with nozzles. A diaphragm is laminated above the liquid chambers and a piezoelectric vibrator is provided on the top portion of the diaphragm. Japanese Laid-Open Patent Application 2-219654 also relates to a production method for a liquid spray head that forms nozzles on a semiconductor substrate. In such a method, a dry film is adhered on the semiconductor substrate, and a diaphragm, a lower electrode, a piezoelectric film and an upper electrode are laminated thereon. The dry film is then removed by conventional means to complete this process.
In U.S. Pat. No. 4,312,008, a liquid spray head is fabricated by providing liquid paths formed in a substrate surface and liquid chambers which pass through the substrate. A substrate is adhered to both surfaces of the substrate and a piezoelectric element is provided thereon.
The Torii, et al. reference merely relates to the use of platinum for the lower electrode of a PZT thin film.
In Japanese Patent Publication No. 4-43435, an electrode formation method for a piezoelectric thin film is discussed in which a metal thin film base and a platinum film are formed on an insulating thin film. The films are heated at a temperature that causes the surface of the platinum thin film to become uneven due to crystal grain growth.
In Japanese Laid-Open Patent Application 3-124450, submitted by the inventors of the present invention, a production method for a liquid spray head is disclosed in which nozzles are formed from one surface of a monocrystalline silicon substrate. As disclosed therein, a p-type monocrystalline silicon is grown by epitaxy and a piezoelectric element is formed on the other surface of the monocrystalline silicon substrate. The p-type silicon layer and the monocrystalline silicon substrate are then etched, and the liquid chambers, a cantilever and center type diaphragms are formed therein.
However, the above prior art liquid spray heads, their component elements and their production methods have various deficiencies, as explained hereinbelow.
In the Japanese Patent Publication No. 62-22790, it is clear that though the thickness of the component elements is not clearly specified, in the embodiments the thickness of the PZT, tp, is believed to be 50 .mu.m and the diaphragm thickness, tv, is believed to be from 50 to 100 .mu.m. Accordingly, it is apparent that Japanese Patent Publication No. 62-22790 could not teach or suggest that the sum of tp+tv should be less than about 10 .mu.m. However, if tp+tv is about 100 .mu.m, as suggest by this reference, the amount of deformation in the diaphragm when a voltage is applied to the PZT is small and inadequate to reliably eject ink from such a liquid spray head. This is a result of the thicknesses of the diaphragm and the PZT layer being too large. Thus, in order to sufficiently deform the volume of the liquid chamber to facilitate the spraying of liquid, a round liquid chamber with about a 2 mm diameter is said to be required. However, in order to increase the resolution in such an apparatus, a planar configuration results in which the liquid chamber pitch is greater than the nozzle pitch as described therein. Such an arrangement results in poor surface area utilization. That is, the planar size of a liquid spray head with seven nozzles is about 20 mm.times.15 mm. As such, if the number of nozzles is increased, not only does the planar size becomes larger, but the speed of the liquid spray operation decreases significantly because the liquid paths linking the liquid chambers and nozzles becomes longer and greatly increases the liquid path resistance.
Moreover, in a method for making such a liquid spray head, a thin diaphragm is fabricated at a position corresponding to the liquid chambers and a PZT layer is formed above the diaphragm. However according to experiments performed by the inventors, when tp+tv was made thinner than specified, e.g., tp substantially equal to 3 .mu.m and tv substantially equal to 1 .mu.m, and the PZT layer was formed after fabricating the liquid chambers and diaphragm, the liquid spray head exhibited sag, wrinkles, breaking, etc., during the production process. This resulted in significantly reduced production yields of the liquid spray head.
Referring to Japanese Laid-Open Patent Application 2-219654, the nozzles in this reference are formed by machining the planar oriented (100) Si substrate. For example, when the nozzles are formed by anisotropic etching of the (100) Si substrate to a thickness of about 300 .mu.m, even though the nozzle dimension is 30 .mu.m square, the angular relationship with the (111) surface which has a slow etching rate unavoidably results in an opening about 400 .mu.m square on the opposite substrate surface. Therefore, it is difficult to make the nozzle pitch less than 400 .mu.m and, thus, the highest resolution possible is only about 60 dots per inch (dpi). That is, it is impossible to increase the density of the nozzles on the liquid spray head in an apparatus according to Japanese Laid-Open Patent Application 2-219654.
Further, as discussed therein, the piezoelectric film and upper and lower electrodes are both larger than the liquid chambers. Accordingly, in such a configuration, it is difficult at best to efficiently deform the diaphragm and spray liquid when voltage is applied to the piezoelectric film. Also, this reference is silent as to the size or thickness of the piezoelectric film, the upper and lower electrodes and the liquid chambers required to efficiently spray liquid.
Finally, in Japanese Laid-Open Patent Application 2-219654, a single SiO.sub.2 layer is used as the diaphragm. As will be understood by one of ordinary skill in the art, SiO.sub.2 has a small Young's modulus of approximately 10.sup.10 N/m.sup.2. Accordingly, when a piezoelectric thin film is formed above the SiO.sub.2 layer and the piezoelectric thin film is deformed laterally by applying a voltage, although it extends a fair distance laterally, its longitudinal deformation is not very great. That is, when one SiO.sub.2 layer is used as the diaphragm, it is impossible to efficiently deform the diaphragm and reliably spray liquid when voltage is applied to the piezoelectric film. Japanese Laid-Open Patent Application 2-219654 is silent regarding the diaphragm characteristics or material required for efficiently spraying a liquid.
U.S. Pat. No. 4,312,008, fails to discuss a configuration in which a piezoelectric crystal is affixed to the top of the diaphragm. U.S. Pat. No. 4,312,008 discusses an embodiment of attachment of the piezoelectric crystal by means of an indium-based solder. As is apparent, the piezoelectric element being used is thicker than disclosed in Japanese Patent Publication No. 62-22790. Therefore, as in Japanese Patent Publication No. 62-22790, the nozzles essentially cannot be fabricated with a sufficiently high enough density. U.S. Pat. No. 4,312,008 also discusses, when using anisotropic etching to form the liquid paths, that the path shape is determined by the surface orientation of the Si substrate and cannot be freely selected. For example, when (100) Si is used, the cross-sectional shape of the liquid path is an inverted triangle, while when (110) Si is used, the cross-sectional shape of the liquid path is rectangular. Such cross-sectional shapes have various deficiencies. More specifically, when the liquid path is an inverted triangle, bubbles readily build up which results in poor quality printing. On the other hand, if the liquid path is a rectangular, the depth is difficult to control, and deviations occur in the liquid spray characteristic.
Further, undercut etching unavoidably occurs where the liquid paths and liquid chambers intersect, which results in an irregular intersection and an inconsistent liquid spray characteristic. In addition to this, since two substrates for sealing the Si substrate and two attachment processes are required in this conventional example, the production process is more complicated and production costs are increased.
In Japanese Journal of Applied Physics, Vol. 30, No. 12B, December 1991, pp. 3562-3566, Torii, et al., a platinum film is formed directly on SiO.sub.2 as the lower electrode of the PZT film. However, in this type of configuration, it is well known that there is a problem with the bond between the silicon oxide and the platinum. Experiments conducted by the inventors confirmed that separation occurred between the silicon oxide and the platinum in heat treatment during or after PZT film formation or during operation after completion. Also, as discussed in Japanese Patent Publication 4-43435, it is known that titanium can be introduced between the platinum and the insulating material to improve the adherence between the silicon oxide or other insulating material and platinum and thereby solve the above problem. However, protrusions occur in the platinum surface in the heat treatment during or after formation of the PZT film and this lowers the breakdown voltage of the PZT film. As a result, spray heads fabricated in this fashion are somewhat more unreliable.
In Japanese Laid-Open Patent Application 3-124450 a configuration is shown in which the etching solution automatically circulates to the surface on the side facing the piezoelectric element when anisotropy etching of the monocrystalline silicon substrate is performed. This configuration results in side etching of the piezoelectric element by the anisotropic etching solution, e.g., potassium hydroxide aqueous solution, of the monocrystalline silicon substrate. Spray head fabricated in accordance with this process, have a lower yield.